The present invention relates generally to ultrasonic vibratory welding and, more particularly, to apparatus for preventing step-edge shearing during such welding operations. Step-edge shearing is a surface quality problem which, heretofore, had been a serious problem in such operations.
Ultrasonic vibratory spot welding processes for joining together two or more similar or dissimilar materials have been used for a number of years. Until recently, however, such methods were limited to use on thermoplastics, non-woven fabrics and metals where weld strength and integrity were not particularly important. This limitation was due, in large measure, to the problems associated with the ultrasonic welding methods employed, most of which were in prototype stages. In those instances when weld strength and weld integrity were important, i.e., when joining together structural aircraft panels and the like, resistance spot welding techniques were used.
Ultrasonic spot welding procedures have recently demonstrated strong potential for improved sheet metal asembly at reduced cost when compared with resistance spot welding and adhesive bonding techniques. Early studies have indicated that weld effected using prototype ultrasonic welding equipment such as, for example, a Sonobond M-8000 ultrasonic spot welder, were superior to welds produced using conventional resistance spot welding procedures. These early trials indicated that for virtually any material combination, an ultrasonically produced spot weld has an ultimate yield strength of more than 2.5 times that of a weld produced using resistance spot welding equipment. Further tests have indicated that ultrasonically produced spot welding can be accomplished with a 75% time and cost savings over conventional adhesive bonding techniques. Until now, however, ultrasonic spot welding for large structural metal parts was not possible in a production environment because of the numerous problems associated with the procedures.
Ultrasonic vibratory welding is a metallurgical joining technique which utilizes high frequency vibrations to disrupt the surface films and oxides and which, therefore, promotes interatomic diffusion and plastic flow between the surfaces in contact without any melting of the materials. Briefly stated, the ultrasonic welding process consists of clamping or otherwise securing together the workpieces under moderate pressure between the welding tip and a support anvil and then introducing high frequency vibratory energy into the pieces for a relatively short period of time, i.e., from a fraction of a second to a number of seconds. In many instances, the pieces to be welded are also adhesively bonded together by the insertion of an adhesive bonding agent between the juxtaposed pieces before welding which results in a high strength joint with superior static and fatigue properties.
One example of an ultrasonic spot welder particularly adapted for use on structural metal workpieces is the Sonobond Model M-8000 ultrasonic spot welder marketed by Sonobond Corporation of West Chester, PA. This welder includes a transistorized, solid state frequency converter which raises standard 60 Hz electrical line frequency to 15-40 kHz and then amplifies the output. The high frequency electrical power travels through a lightweight cable to a transducer in the welding head where it is converted to vibratory power at the same frequency. The vibratory power is, thereupon, transmitted through an acoustic coupling system to the welding tip and then through the tip into and through the workpieces, with the vibratory energy effecting the weld.
The Sonobond M-8000 ultrasonic spot welder includes a wedge-reed transducer coupling system which transmits lateral vibrations of a perpendicular reed member attached to it so that the welding tip at the lower end of the reed executes shear vibrations on the surface of the workpieces. The transducer includes piezoelectric ceramic elements encased in a tension shell assembly and operates at a nominal frequency of 15 kHz. A solid state frequency converter with a transistorized hybrid junction amplifier powers the welder. The converter operates at a nominal frequency of 15 kHz with a power output variable up to about 4000 RMS RF watts. The welder may be tuned to a precise operating frequency. The frequency converter includes a wide-band RF power measuring circuit which samples output power and detects forward power and load power based on the principle of bi-directional coupling in a transmission line. The signal is processed electronically to provide true RMS values which are selectively displayed on an LED panel meter as either the forward or load power. Forward power is the output of the frequency converter delivered to the transducer in the welding head while load power is the transducer drive power acoustically absorbed by the anvil. The difference between the two readings is the reflected power induced by the load impedance mismatch and is minimized during the welding operation by impedance matching techniques.
Step-edge shearing is a surface quality problem which has, oftentimes, been experienced during ultrasonic welding. The use of adhesive interlayers between the workpieces to be welded tends to cause a step-type indentation around the edge of the welding tips and/or the anvils of the ultrasonic welding apparatus which typically are dimensioned such that the edges of the anvils and/or the welding tips dig or otherwise cut into the sheets being welded together. In many ultrasonic welding applications, the workpieces to be joined are not only welded but are also adhesively bonded together by the insertion of a film adhesive between the workpieces prior to welding. Step-type indentations become a particularly serious problem when the workpieces to be welded together are relatively thin because it has been found that thin sheets are particularly subject to shear deformation due to the resistance of the film adhesive to flow and, further, due to the shear deformation enhancement of the ultrasonic energy being applied.
It will, of course, be appreciated that this step-edge effect is highly undesirable from both a quality and stress concentration point of view. In an effort to overcome the problem, larger diametered anvils having relatively shallow curvatures were investigated. While, in theory, apparatus using such anvils have reduced sharp step-edges, they were generally unacceptable since they prevent close juxtaposition of the spots to other structures that may be on the assembly.
Against the foregoing background, it is a primary object of the present invention to provide apparatus for overcoming the sharp step-type indentations encountered during ultrasonic welding.
It is another object of the present invention to provide such apparatus which does not unduly limit the applicablity of the ultrasonic welding operation.
It is yet another object of the present invention to provide such apparatus wherein a shield is provided about the welding tip and/or the anvil which is effectively isolated from the mechanical energy which passes through the welding tip and/or the anvil.
It is still another object of the present invention to provide such apparatus wherein the shield is used to apply pressure to the workpieces about the edges of the welding tip and/or the anvil.